Well, I was a head coach of a high school team and called the plays. It IS difficult to prepare, then adapt to game situations, especially if the opposing coach is smarter, better prepared or simply has a better team. I consider myself a failure as a coach, yet still curse guys like Hackett for their calls, because there ARE coaches around like Belichick who are simply better prepared and better at adapting.
For example, BB will figure out the opposition's weak link on D and attack the crap out of it. Other coaches don't seem to do this. On defense, Belichick will figure out the QB's weakness and which OLineman can be beat and attack that spot. In football, coaching is about 50% of the game and let's face it, guys like Jimmy Johnson are smarter than guys like Marv Levy, let alone Douglas Marrone ...
or me!